Vehicle drive control apparatuses for adjusting a drive torque in response to an operating state of a vehicle having a plurality of motors are known in the art, as disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2003-333707 (JP 2003-333707 A).
The vehicle drive control apparatus disclosed in JP 2003-333707 A has a motor for driving left and right front wheels and a motor for driving left and right rear wheels in a vehicle, and controls the two motors (drive sources). A required torque demanded by the operator is distributed to the two motors in a predetermined distribution ratio. Each of the motors outputs a drive torque according to the distribution ratio. The required torque is determined from a vehicle speed and an accelerator position on the basis of a predetermined required torque setting map.
There are vehicles in which separate motors drive each of the four wheels. The technique described in JP 2003-333707 A can also be considered for application in vehicles of this type; i.e., a required torque is set according to a required torque setting map, and the required torque is distributed to each motor individually in a predetermined distribution ratio.
However, the maximum torque able to be output by each motor alone (individual critical torque) changes according to the rate of rotation of each motor; i.e., according to the vehicle operation state. The required torque merely is distributed to the plurality of motors in a predetermined distribution ratio. In a case in which the required torque distributed to a single motor exceeds the individual critical torque, the distributed required torque cannot be output. Complications are accordingly presented when setting a required torque suitable to the vehicle operating state.
In a hypothetical example, the rates at which the left and right motors rotate are the same (when the vehicle is moving directly forward), and the following circumstance can be considered when the required torque setting map is created.
When the vehicle turns, the rate of rotation of the outer turning wheel will be greater than the rate of rotation of the wheels when the vehicle moves directly forward. The rate of rotation of the motor for driving the outer turning wheel will also be greater; therefore, the individual critical torque able to be output by the motor will decrease. Consequently, the required torque cannot always be output up to the maximum value. Therefore, it is possible that the torque may not increase from an intermediate position before the accelerator has been fully depressed. The same is true when the wheels are rotating while disengaged from the motor.
In another hypothetical example, the rates of rotation of the left and right motors are different (when the vehicle is turning), and the following circumstance can be considered when a required torque setting map is created to correspond with the rate of rotation of the outer turning wheel.
When a vehicle is moving directly forward, the rate of rotation of the wheels is less than the rate of rotation of the outer turning wheel when the vehicle is turning. Therefore, the individual critical torque able to be output by a motor when the vehicle is moving directly forward should be greater than the individual critical torque able to be output by a motor for driving an outer turning wheel.
However, the required torque setting map is created in correspondence with the rate of rotation of the outer turning wheel, as described above. Consequently, the required torque setting may end up being smaller than the individual critical torque when the vehicle is moving directly forward. Therefore, the torque may end up being limited before reaching the individual critical torque, regardless of the fact that when the vehicle is moving directly forward, inherently, the motor is actually capable of outputting a value that is greater than the maximum required torque of the required torque setting map.
Complications will thus be presented when setting the individual critical torque for each of the motors to the optimal value according to the vehicle operating state; therefore, the operator may experience an unpleasant sensation.